This invention relates to an improved method for separating fluoride ion from an aqueous solution thereof and is particularly pertinent to the removal of fluoride ion when the concentration thereof is about 20 ppm or less, the method being effective for reducing the fluoride ion to a level as low as 0.20 ppm.
Conventional methods of removal of fluoride ion from raw water are expensive and ineffective where the fluoride ion concentration is 8 ppm or less. According to Hillyer et al in U.S. Pat. No. 2,914,474 the fluoride content of the effluent from a triple superphosphate plant can be reduced to as low as 8 parts per 1,000,000 by bringing the pH of the effluent to above 6.0 by the addition of calcium carbonate. In general, the fluoride content is reduced only to about 20 ppm but under optimum conditions can be brought to as low as 8 ppm. Kanai in U.S. Pat. No. 3,414,497 has taught the addition of CaF.sub.2 to a polluted aqueous liquid and electrolysis thereof with specially-coated electrodes to clarify the solution. The positive electrode is coated with potassium compounds and compounds of silicon, barium, lead or calcium and the negative electrode is enclosed in a bag containing F.sup.- and I.sup.- compounds. It should be noted that the objective is to clarify the polluted water and not to remove fluoride from the solution. Further to this point, Kanai refers to the use of fluoride-releasing compounds such as sodium fluoride and lithium fluoride as alternatives to calcium fluoride.
Adamson in British Pat. No. 775,143 has taught a method of reducing the fluoride content from 24.5 ppm to 2.4 ppm or lower by boiling the water with a mixture of magnesium oxide or magnesium hydroxide with ammonium chloride. The reaction is rapid when the solution is boiled; however, such a procedure is prohibitively expensive where large quantities of water are to be treated. Adamson states that the process is effective even in the cold provided that a sufficiently long reaction period is allowed. Again, such a procedure can be impractical where large quantities of water are to be treated.
As is evident then, a practical method for reducing the fluoride content of water to an acceptable value is greatly to be desired. This is especially the case where expense is a significant factor and where the final fluoride content must be substantially below 1.0 ppm.